Flexible thin film solar cell

ABSTRACT

A clear thermal emissive coating, such as clear polyimide, is deposited directly upon a thin film solar cell forming a flexible thin film solar cell. The thin film solar cell can be deposited on another thermal emissive coating used as a substrate during thin film solar cell semiconductor processing so that resulting flexible thin film solar cell can be illuminated on the top side and eject heat from both sides suitable for forming a solar cell array over a curved surface such as a power sphere nanosatellite with thermal regulation.

REFERENCE TO RELATED APPLICATION

The present application is related to applicant's application entitled“Integrated Solar Power Module” Ser. No. 09/649,726, filed Aug. 28, 2000now U.S. Pat. No. 6,300,158.

The present application is related to applicant's application entitled“Power Sphere Nanosatellite” Ser. No. 09/520,120, filed Mar. 6, 2000 nowU.S. Pat. No. 6,284,966.

The present application is related to applicant's copending applicationentitled “Power Distribution System” Ser. No. 08/281,653, filed Mar. 30,1999 allowed.

FIELD OF THE INVENTION

The present inventions relate to the field of thin film devices andprocesses. More particularly, the present inventions relate to thin filmsolar cells and thin film flexible circuit boards, and methods ofmanufacturing the same.

BACKGROUND OF THE INVENTION

There has been recently interest in the conceptual designs formicrosatellites and nanosatellites for various space missions deployed,for example, in low earth orbits. These microsatellites andnanosatellites require the collection of sufficient power for onboardinstruments. These microsatellites and nanosatellites are typically lowin weight and low in volume and have a limited amount of surface areafor power collection. Because the overall surface area of amicrosatellite or nanosatellite is small, body-mounted solar cells maynot provide enough power for the on-board instruments. Traditional rigidsolar arrays necessitate larger satellite volumes and weights and alsorequire extra apparatus for pointing the spacecraft solar arrays.

The limited surface area of the microsatellites or nanosatellites is thepower choke problem where insufficient energy is collected. Onepotential solution to the power choke problem is the use of a sphericaldeployable power system having a spherical outer surface covered withsolar cells offering a high collection with low weight and low stowagevolume, while eliminating the need for a solar array pointing mechanism.For powering spacecraft, the collection of solar energy requires theexposure of solar cells to sunlight. The thin film solar cell must becapable of maintaining thermal equilibrium by radiating all thermalenergy absorbed while operating in a space vacuum environment. Solarcells absorb nearly 90% of the incident sunlight and convert a smallportion, for example 5-20%, of that energy to electricity. To maintainthermal equilibrium, the solar array has thermal properties that allowrejection of the total solar input as thermal radiation. The solar arraydesign uses materials that have the appropriate thermal radiationcoefficients to allow thermal equilibrium to be reached at a temperaturethat is optimum for the efficient operation of the solar cells.

The power sphere is a curved electrical power system. However, modernsolar cell panels are typically fabricated using a plurality of rigidsolar cell panels unsuitable for flexible forming about curved surfaces.For thermal radiation of heat absorbed from incident solar radiation andfor the collection of energy through solar illumination, typically arigid solar cell would be bonded using adhesives to a thick transparentcover glass. This same basic fabrication methodology has beentransferred to the production of thin film solar arrays by using anadhesive to bond a transparent polymer on the top of the thin film solarcell. The adhesive is subject to damage and failure through thermalcycling, radiation and solar ultra violet illumination.

Thin film solar cells have been deposited on kapton polyimide forming anintegral flexible thin film solar cell that is then bonded betweenopposing sheets of polymer (i.e. Tefzel) having the required transparentand thermal emissive properties. These thin film solar cells have beenused in terrestrial application but suffer from the use of bondingadhesive completely covering the surfaces of the sheet polymer tefzelused for terrestrial environmental protection.

Thin film solar cells could be bonded using the adhesive to flexiblethin film circuit boards. Traditional flexible printed circuit boardsare fabricated by laying copper circuitry down on both sides of aflexible substrate and then using adhesive to bond subsequent pairs ofcircuits or alternating a polymer material layer with the coppercircuitry and laminating each pair of layers to bond the pairs. Themultiple layers of polymer material bonded together with an adhesivewith each layer having a different coefficient of thermal expansionwould induce differential mechanical stress between the various layersat different operating temperatures. This arrangement is unsuitable forflexible thin film solar arrays because the adhesive layer may bedamaged by temperatures required for deposition of the thin film solarcell on the flexible circuit, because the adhesive layer adds weight andmight result in relatively thick films, and because the multiple layersof polymer material film and adhesive would result in a film withdifferences in coefficients of thermal expansion that would inducedifferent mechanical stresses in the composite material at differenttemperatures during solar illumination cycles. The multiple layers ofpolymer material and adhesive layers significantly increases the massand complexity of the power sphere. For commercial terrestrial uses,this problem of alternate layers of adhesive and polymer might bemanageable, but for the space environment this delaminating problem ismuch greater due to the wide temperature extremes that the power spherewill be exposed to during different parts of each orbit. In addition,the total number of thermal cycles that a low earth orbiting satelliteexperiences on a daily basis is far greater than one would expect formost terrestrial applications.

The basic architecture of the power management and control system forthe power sphere requires regulation of each individual solar cellmounted on the surface of the sphere. This power management and controlsystem has not been integrated with flexible thin film solar cellssuitable for flexible forming about a small curved surface such as theexterior of a nanosatellite or microsatellite. These and otherdisadvantages are solved or reduced using the invention.

SUMMARY OF THE INVENTION

An object of the invention is to provide a flexible space qualified thinfilm solar cell having a thermal emissive layer for heat rejection.

Another object of the invention is to provide a flexible circuit boardhaving multiple layers of deposited thin films adhered together withoutthe use of bonding adhesives.

Yet another object of the invention is to provide a flexible solar cellpower module having a top thermal emissive layer and a bottom flexiblecircuit board integrally fabricated using thin film processes.

The present inventions are directed to a flexible thin film circuit,such as thin film solar cells, and methods of manufacturing the same. Inorder to reduce mass of the microsatellite, an integrated thin filmsolar power module is used. The thin film solar cell with associatedinterconnects and power processing electronics for regulating the solargenerated electricity are integrated into the polymer material substrateon which the thin film solar cell is fabricated. The thin film solarcell is fabricated on a flexible printed circuit board. The flexibleprinted circuit board is made of a polymer material with the requiredcopper traces for the electronic circuit embedded within the thicknessof the polymer film.

In order to simplify the wiring harness of a power sphere, a preferredlocation for the required power regulation electronics required for eachsolar cell is on the backside of the substrate for the solar cell. Thecircuit traces for the power processing electronics are not bonded toflexible thin film solar cells, but are integrated into thin film layerby deposition of circuit traces within thin films forming a thin filmcircuit board, which is also the substrate for the thin film solar cell.The integrated thin film assembly can be used in the power sphere withreduced mass and wiring. The flexible thin film solar power module isconnected to a simple two wire bus eliminating the need to runindividual wires from each solar cell to a central grouping of powerprocessing electronics.

The integrated multiple layer flexible thin films in an integratedcircuit board eliminates the need for multiple adhesive layers betweenmultiple polymer layers thereby eliminating large variants in thermalcoefficient of expansion and difference in thermal emissivity. Thefabrication of multiple thin films upon one another without the use ofadhesives enables the manufacture of a flex circuit board that has onlytwo components, which are the polymer material and the embedded metalfor the circuit traces. This improved flex circuit board can befabricated with extremely thin layers of polymer material on which themetal circuit traces are deposited between deposition of subsequentlayers of polymer material. The resultant flexible circuit board can beutilized as the substrate for depositing the thin film solar cell.Another top thin film layer of clear polymer can be deposited on top ofthe solar cell thereby providing proper thermal emissivity without theuse of an adhesive. Hence, the solar power module comprising a topthermal emissive layer, center thin film solar cell and the bottom flexcircuit is an integrated flexible thin film solar cell power module.Additionally, the integrated flexible solar cell power module could befurther enhanced with mounted discrete power electronic components onthe backside of the flexible circuit board assembly for providingnecessary electronic processing control. These and other advantages willbecome more apparent from the following detailed description of thepreferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a thin film solar cell.

FIG. 2 depicts a thin film solar cell power module with an integratedthin film circuit board.

FIG. 3 is a flow diagram of processing steps for manufacturing a thinfilm solar cell power module.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention is described with reference to thefigures using reference designations as shown in the figures. Referringto FIG. 1, a thin film solar cell and a transparent polyimide layer forma flexible thin film solar cell. The transparent polyimide layer ispreferably a thermal emissive transparent polyimide layer enabling solarillumination of the solar cell which provides thermal emissivity ofheat. In various preferred forms, the flexible thin film solar cell mayfurther comprise an opaque polyimide layer as a covering orinterconnection layer. The opaque polyimide layer can be disposed on thethin film solar cell to improve strength or for integrating additionalelectronic devices, circuits and conduction traces.

The covering layer of polyimide can be deposited using a solvent spinmethod. Polyimide is dissolved in a solvent, such as Dimethylacetamidein a concentration of 15% weight solids, and spun on the thin film solarcell that is then baked at a temperature of 180 degrees Centigrade, for12 hours. A cover layer can be as thin as 0.1 microns, but thick layersare more desirable. The cover layer must be at least two Mils thick toobtain the required thermal emissive properties. The thickness of thelayer can be grown by applying additional coats of the polyimide in thesolvent. The solvent penetrates into the existing layer, and when baked,the solvent evaporates leaving an additional amount of polyimide on theexisting layers. As a result, the covering layer is a homogenous uniformlayer without any interface transitions between successive layers. Inthis manner, the thickness of the cover layer can be controlled to adesired thickness.

The method of manufacturing an integral thin film solar cell comprisesthe steps of depositing a thin film solar cell on a sheet of polyimidebecoming a thermal emissive covering layer for the thin film solar cellfor communicating solar illumination to the thin film solar cell and forheat rejection. Next, a second covering layer is deposited the thin filmsolar cell. The second covering layer is preferably an opaque polyimideor a polymer.

Referring to FIG. 2, an integrated solar power module includes the thinfilm solar cell. Thin film solar cell materials typically have lowthermal emissivity and thermal emission is necessary for regulatingheat. The thin film solar cell is integrated with a thermal emissivetransparent polyimide layer functioning as cover glass. The thermalemissive transparent layer may be deposited on the thin film solar cell.The thin film solar cell can function as a substrate for deposition ofthe polyimide layer that offers thermal emissivity that can be used toreject thermal energy as thermal radiation yet enable solar illuminationof the thin film solar cell. A plurality of solar cells can beintegrated together as a flexible thin film solar array that can be usedin a space vacuum environment where the only method of rejecting thermalenergy is through thermal radiation. The addition of a transparentplastic type material with high thermal emissivity such as clearpolyimide solves the thermal problem and protects the cell from spaceradiation. The top thin film clear polyimide is deposited directly onthe amorphous silicon solar cell using polymer-manufacturing processeswithout the use of bonding adhesives.

The thin film solar cell power module further includes a solar cell topcontact and a solar cell bottom contact for interconnecting a pluralityof solar cells, not shown, to form an array of solar cells. The solarcell power module includes a top contact layer for interconnecting theplurality of solar cells for forming a flexible solar array panel for asolar cell array including the solar array and the covering thermalemissive transparent polyimide layer. An insulator is used forinsulating the top and bottom solar cell contact layers for extending avertical feed through from the top contact layer into theinterconnection layer.

A polyimide interconnect layer may be deposited on the backside of thethin film solar cell. In the preferred form, the polyimideinterconnection layer functions as a substrate on which is deposited thethin film solar cell as well as the top and bottom contacts and thecovering thermal emissive layer. The interconnected layer is built upfrom a plurality of layers deposited during respective deposition stepsbetween which lateral and traverse embedded interconnection traces canbe deposited. The lateral and traverse embedded traces extendhorizontally through the polyimide interconnection layer. Verticaltraces, including a top solar cell trace, bottom solar cell trace andvertical embedded traces may also be formed in the polyimideinterconnection layer. The vertical traces are formed by creating avertical feed through in the interconnection layer and depositingconducting metal within the feed through. The horizontal and verticaltraces can be manufactured to form any arbitrary three-dimensionalinterconnection matrix within the interconnection layer. Theinterconnection layer can be any suitable insulating polymer film, butmust also be a thermal emissive layer as well for heat rejection. Anelectronic component, such as power converters used in a solar cellpower regulation system, can be deposited or bonded onto the back sideof the interconnection layers and then connected to the interconnectionmatrix using component leads or deposited metal etch runs. Even thoughthe devices may be bonded, the bonded area under the devices isrelatively small and segmented. Therefore, differences in thermalexpansions at the adhesive layer will not lead to failure duringtemperature cycling. As such, the interconnection layer and integratedmicroelectronic component can form any arbitrary flexibleinterconnection circuit board with and without microelectroniccomponents. The solar cell power module is an integrated deviceincluding the thermal emissive covering, the solar cell, interconnectionmatrix in the interconnection layer and microelectronic powerdistribution components. A plurality of solar cell power modules formedintegrally together would form a complete solar array power system thatis flexible for forming about curved surfaces, such as a power spherenanosatellite.

Referring to all of the Figures, and more particularly to FIG. 3, thesolar cell power module can be fabricated using semiconductor andpolymer type manufacturing processes in a batch mode or continuousroll-to-roll mode using economy of scale for mass production. The solarcell power module makes use of a substrate interconnection layer thatpreferably has high thermal emissivity. Thin film solar cell materialmay be deposited directly on the substrate interconnection layer duringfabrication processes. The completed thin film solar cell is coveredwith the flexible transparent cover that has the required thermalemissivity. The transparent cover material may be deposited on the solarcell by directly depositing the layer of clear polyimide directly on thetop layer of the amorphous silicon solar cell. This process eliminatesthe need for adhesives to bond the transparent cover to the solar cell.

In the preferred form, a release structure, such as a stiff sheet ofTeflon, is first used as a substrate on which is deposited the polyimideto form the interconnection layer in repeated applications so as to growthe thickness of the interconnection layer. In between successivedepositions of the polyimide of the interconnection layers,interconnection layers having horizontal lateral and traverse metaltraces can be deposited for burying the traces within theinterconnection layer. A metal, such as copper, is deposited on thepolymer material interconnection layer over which another layer ofpolymer layer is deposited. This process is repeated to fabricatemultilayer flexible printed circuit boards. After depositing all ofinterconnection layers, the release structure is removed with theresulting interconnection layer now functioning as a substrate for thinfilm solar cell deposition including the deposition of the top andbottom solar cell contacts. In a series of conventional solar cellprocessing steps, a thin film solar cell is deposited on the flexibleprinted circuit board. Feed throughs are then preferably drilled, oralternatively etched into the interconnection layer and the embeddedvertical traces are deposited into the cavity of the drilled feedthrough. The vertical traces connect with the horizontal traces and thetop and bottom solar cell contacts. Preferably, next the top coveringglass thermal emissive polyimide layer is deposited directly on the thinfilm solar cell including the top solar cell contact layer. Themicroelectronic device is bonded to the backside of the interconnectionlayer. Preferably, the mounting of discrete electronic components on thebackside or depositing semiconductor devices on the backside occursafter depositing the thin film solar cell. Connection leads are bondedto the microelectronics device and interconnection layer for electroniccoupling the device to the solar cell. As such, an integrated flexiblethin film solar cell power module is formed comprising a thermalemissive coating directly deposited upon the thin film solar cellwithout the use of bonding adhesive, and the interconnection layer andmicroelectronic device form a flexible circuit board. The board andcoated thin film solar cell can be extended during fabrication to form aflexible solar cell array power system.

The thin film solar cell can be made by differing processes. Forexample, the thin film solar cell can be deposited directly on a sheetof rolled kapton polyimide with the cover layer then deposited on thethin film solar cell. The kapton rolls are available in 0.08 to 2.0 Milsin thickness. While kapton polyimide may be used, other polymers withtransparent and thermal emissive properties could be used as either thecovering layer or the interconnection layer.

The integrated solar cell module can be made by differing processes aswell. For example, the thin film solar cell can be deposited on a 0.5 to2 mil thick polyimide substrate and then the interconnection layer canbe deposited on a back side of the polyimide substrate of the thin filmsolar cell. Feed through holes can be made in the interconnection layerfor forming the vertical traces and for bonding of the microelectronicdevices and bonding wire interconnections. Finally a transparent thermalemissive covering layer is then deposited on the thin film solar cell.

As may now be apparent, the flexible integrated solar power moduleconsists of a flexible printed circuit board, that is, theinterconnection layer, a flexible thin film solar cell with a flexibletop thermal emissive coating formed without the use of bondingadhesives, and associated power regulating electronics that may bebonded or deposited on or within the interconnection layer. The flexibleprinted circuit board being used as the substrate for a thin film solarcell deposition on one side, and used for mounting electronic circuitcomponents on the other side. In the preferred form, both the top clearcovering layer and the bottom interconnection layer are both made ofclear polyimide offering front and back heat rejection. The clearpolyimide interconnection substrate offers additional heat rejection.The thermal design for a typical solar power sphere may require athermal emissivity of approximately 0.8 for both the front and backsurfaces of the flexible thin film solar cell.

The flexible thin film solar cell, the flexible thin film solar cellpower module, the flexible thin film solar array power system, and theflexible circuit board can be used for both terrestrial commercial useand for use in a space vacuum environment. The flexible thin film solarcell power module or solar array power system can be used, for example,for collecting power in a solar power sphere concept as the electricpower system for microsatellites and nanosatellites. This improved powersphere with flexible solar array panels is well suited for integrationin of microsatellites and nanosatellites in low earth orbits thatrequire the collection of sufficient power for onboard instrumentswithin a low weight and low volume spacecraft. Because the overallsurface area of a microsatellite or nanosatellite is small, body mountedflexible solar cells are capable of providing sufficient power without apointing apparatus as the solar illumination of the power sphere isconstant regardless of the attitude of the solar power sphere. Hence,the solar power sphere with the surface mounted flexible thin film solarpower modules would offer a high collection area relative to the lowweight and low stowage volume of the microsatellite while eliminatingthe need for a solar array attitude pointing mechanism. The sphericalshape collects the same amount of solar energy for any angular positionof the sun relative to the satellite. When in sunlight, the constantenergy absorption provides stable electrical power generation and astable, moderate thermal environment for the enclosed spacecraftpayload, independent of satellite attitude. During eclipse periods, thesphere also acts as a thermal radiation barrier, minimizing thetemperature drop of the enclosed satellite payload during the eclipse.The solar power sphere converts sunlight into direct current electricitythrough the use of the thin film flexible solar cell power modulemounted on the deployable spherical structure.

The present inventions include the thin film solar cell with the thermalemissive coating directly deposited onto the thin film solar cell. Theinventions also include the interconnection printed circuit board, aswell as an integrated solar power module that can be extended into aflexible solar array power system. The solar power sphere thus canutilize a deployable solar cell array power system formed to theexterior curve surface offering power collection and thermal regulationof the microsatellite or nanosatellite. Those skilled in the art canmake enhancements, improvements, and modifications to the invention, andthese enhancements, improvements, and modifications may nonetheless fallwithin the spirit and scope of the following claims.

What is claimed is:
 1. A method of manufacturing an integral thin filmsolar cell array, the method comprising the steps of, depositing thinfilm solar cells on a flexible sheet of polyimide becoming a firstthermal emissive covering layer for the thin film solar cells forcommunicating solar illumination to the thin film solar cells and forheat rejection and forming a flexible sheet of solar cells, applying asolvent based thermal emissive covering layer onto the thin film solarcells of the flexible sheet of solar cells, and curing the solvent basedthermal emissive covering layer into a second thermal emissive coveringlayer for forming the integral thin film solar cell array.
 2. The methodof claim 1 wherein, the second covering layer is polyimide.
 3. Themethod of claim 1 wherein the second covering layer is a polymer.
 4. Themethod of claim 1 wherein, the applying step is a rolling applying stepfor continuous roll-to-roll application of the solvent based thermalemissive covering layer onto the flexible sheet of solar cells.